1. Field of the Invention
This invention relates to optical amplifier systems and more particularity to systems including distributed and discrete Raman amplifiers.
2. Technical Background
Optical amplifiers increase the amplitude of an optical wave through process known as stimulated emission. During this process a pump photon, supplied an optical pump, excites an electron to a high energy level is an optical material. When the signal photon interacts with an excited electron, the electron undergoes the transition to a lower energy level. In the process, the material emits a coherent photon with the same frequency, direction and polarization as the initial signal photon. The two photons can, in turn, serve to stimulate the emission of two additional photons coherently, and so forth. The result is coherent light amplification. Stimulated emission occurs when the photon energy is nearly equal to the atomic transition energy difference. For this reason, the process produces amplification in one or more bands of frequencies determined by the atomic line width. The signal bands are a conventional band (C band) with a wavelength range for approximately 1528 to approximately 1565 nm and a Long band (L-band) with the wavelength range for approximately 1568 nm to approximately 1620 nm.
Long distance communication systems typically use Erbium doped fiber amplifiers between long spans of transmission fiber. A typical configuration for the Erbium doped fiber amplifier includes one or more pump lasers operating at wavelengths of 980 nm or 1480 nm and providing an output which is coupled into the Erbium doped fiber. Erbium doped fiber amplifiers are discrete amplifiers. Such amplifiers are used to provide a sufficient amount of gain in order to compensate for loss of signal in transmission fiber, often requiring multiple high power pumps. Erbium doped fiber amplifiers are used to produce a high power out-going signal, because otherwise, as the signal travels through the transmission fiber, the attenuated signal level would approach the noise level by the time it reaches the next amplifier. Erbium doped fiber amplifiers typically have to be positioned at no more than a hundred kilometers apart from one another, otherwise the signal level would drop down to approximately the noise level before reaching the next EDFA and the next amplifier might not be able to distinguish between the noise and the signal.
In addition, a network provider may not initially need a broad band capability and may, in order to cut the costs down, want to offer only C-band to his clients. However, as the demands on the network system increase, the network provider may want to expand to the L-band. Unfortunately, once a network is layed down, it is expensive to add an L-band capability because this would require the addition of many L-band components.
Discrete Raman fiber amplifiers can be used as amplifying devices that compensate for losses incurred by the signal during its transmission through the transmission fiber. For this purpose, the discrete Raman fiber amplifiers would also be situated between long lengths (typically 40-100 kilometers) of transmission fiber. Unfortunately, an amplifier system with only discrete Raman amplifiers suffer from MPI (multipass interference), from double Rayleigh backscattering and gain saturation from the pump depletion.
The distributed Raman fiber amplifiers are sometimes used in conjunction with the Erbium doped fiber amplifiers. Typical distributed Raman fiber amplifiers utilize typical transmission fiber as the gain medium. However, the signal propagating through the distributed Raman amplifier undergoes distortion and, thus becomes broader due to chromatic effects produced by the fiber. This distorted signal is provided to and is amplified by the Erbium doped amplifier and, contributes to reduction of the signal to noise ratio. In order to compensate for the signal broadening, such an amplifier system typically utilizes a multistage Erbium doped fiber amplifier which has a dispersion compensating fiber between the two amplifying stages of the Erbium doped amplifier. Splicing a dispersion compensating fiber between two amplifying stages of the Erbium doped fiber amplifier introduces about 10 dB center stage loss into the amplifier, which may be overcome by additional pumping power with a resulting increase in cost.